1. Field of the Invention
The present invention is directed to a probe assembly for a metrology instrument used to measure a property of a sample, and more particularly, a probe assembly including a cantilever having a short length to support high bandwidth operation, and configured for ready batch fabrication.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which use a sharp tip and low forces to characterize the surface of a sample down to atomic dimensions. Generally, the tip of the SPM probe is introduced to the sample surface to detect changes in the characteristics of the sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
In an AFM, for example, in a mode of operation called contact mode, the microscope typically scans the tip, while keeping the force of the tip on the surface of the sample generally constant. This is accomplished by moving either the sample or the probe assembly up and down relatively perpendicularly to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Similarly, in another preferred mode of AFM operation, known as TappingMode™ (TappingMode™ is a trademark owned by the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample.
The deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, most often an optical lever system. In such optical systems, a lens is employed to focus a laser beam, from a source typically placed overhead of the cantilever, onto the back side of the cantilever. The backside of the lever is reflective (for example, using metalization during fabrication) so that the beam may be reflected therefrom towards a photodetector. The translation of the beam across the detector during operation provides a measure of the deflection of the lever, which again is indicative of one or more sample characteristics.
One area of continuing SPM development relates to the speed of operation. In this regard, the greater the resonant frequency of the cantilever of the probe of the probe assembly the greater the speed at which the SPM can be operated to acquire sample surface data. One way in which high operational resonant frequencies, and thus improved SPM imaging speed, can be facilitated is by using a probe having a cantilever that is much shorter than the typical length of about 100-400 microns. This is due to the fact that, with a shorter lever, the instrument can be operated at a higher resonant frequency with less noise. Therefore, keeping the same spring constant, one can operate the SPM faster while obtaining high integrity data given a greater signal to noise ratio. Preferably, a probe having a cantilever that is less than 50 microns or even less than 20 microns is preferred for such applications.
One significant drawback associated with using probes having short cantilevers, however, is that for a number of reasons it is very difficult to bulk manufacture probes having cantilevers with such short lengths, i.e., in the sub-50 micron range. In most such processes, the lever is formed, as well as the tip, using micro fabrication techniques that require precise alignment of the manufacturing tools (e.g., photolithography masks, etc.) and precise processing of the probe components, including bonding a substrate to the formed probe prior to dicing the substrate into individual probe assemblies. In the latter regard, when producing short levered probes, it is nearly impossible to accurately control the dicing from the backside of the batch fabricated probe assemblies given alignment inaccuracies in the process. This causes an offset between the edge of the diced substrate and the tip or distal end of the cantilever. This offset cannot be readily controlled. As a result, probe assemblies having profiles such as that shown in FIGS. 1A-1C may result. In particular, for example, FIG. 1A illustrates a probe assembly 15 having a lever 16 with a length about 30 microns, as desired. However, given alignment inaccuracies and related complicating factors during fabrication, the probe assembly 15′ of the next batch may be formed such that the glass substrate 18 bonded to the probe yields no cantilever, such as that shown in FIG. 1B. Finally, in the next batch, as shown in FIG. 1C, the probe assembly 15″ may have a cantilever 16′ with a significantly greater length, such as 60 microns. In sum, the offset from the edge of the substrate to the tip upon bonding and dicing the substrate, hereinafter called the “uncontrollable offset”, yields a cantilever having a length that cannot be predictably controlled, assuming the cantilever is formed at all.
In this regard, two specific types of probes employing silicon nitride cantilevers are shown in FIGS. 2A (glass substrate) and 2B (silicon substrate). In FIG. 2A, a probe assembly 20 includes a probe 21 having a silicon tip 22 and a silicon nitride lever 24 that extends from a glass substrate 26. In this case, the substrate 26 is bonded directly to the silicon nitride that forms and defines the length of cantilever 24. As a result, when batch fabricating such a probe, the uncontrollable offset, “0”, as shown in FIG. 2C, present when the probes are diced operates to limit the manufacturer's ability to produce repeatable probe assemblies having a short, for instance, sub-20 micron, length. More particularly, standard mechanical dicing operations have inherent alignment errors (typically, as much as tens of microns) that, though acceptable for fabricating conventional probes, is unacceptable for fabricating the type of short probes contemplated herein. In FIG. 2B, a self-actuated probe assembly 30 includes a probe 32 having an integrated actuator 34 and a base substrate 35 made of silicon. A cantilever 36 of probe 32 made of silicon extends from actuator 34 defined by top and bottom gold electrodes 38, 40, respectively, and an active element 42, such as zinc oxide. Here again the substrate 35 is bonded directly to the silicon nitride layer, or electrode 38, that defines cantilever 36. In this case, the individual probe assemblies are released with an appropriate etch of the sacrificial silicon. Due to processing limitations, performing this etch with sufficiently high precision to cost-effectively define cantilevers 36 having repeatable lengths in the sub-20 micron scale is generally impossible, as understood in the art. In particular, in this case, the length of the cantilever is typically defined by front-side and back-side etches of the substrate. It is very difficult to control these etches to define short (e.g., sub-20 micron) levers because of orthogonality and parallax considerations, as understood in the art.
In view of the above, the art of scanning probe microscopy was in need of a probe assembly having a short lever, i.e., less than 20 microns, and a corresponding method of batch fabricating the probe such that the length of its associated cantilever can be precisely controlled and batch processed independent of inherent alignment errors associated with fabrication processes in which the probes are either diced or etched. This control of the length of the cantilever should be realized without compromising the physical properties of the probe. For instance, the spring constant must be maintained so that the probe is capable of operating at high bandwidth, thus allowing the SPM to perform high speed imaging.
Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “atomic force microscopy.”